Cellular communication systems are well known in the art for providing wireless communications links between mobile telephone users and stationary users or other mobile users. In typical cellular systems, wireless transmissions from mobile users are received by one of a local network of base station terminals. The base station or "cell" receiving the signal then retransmits it for reception by the intended receive terminals, either directly or via the cellular or local telephone systems.
As cellular communications systems generally rely on line-of-sight communications links, many cells are typically required to provide communications coverage for a large geographic area. The cost associated with providing large numbers of cells, however, may prohibit the use of cellular systems in sparsely populated regions or areas where there is limited demand for cellular service. Moreover, even in areas where cellular service is not precluded by economic considerations, "blackout" areas often arise in traditional cellular systems due to local terrain or weather conditions.
As such, systems that integrate cellular and satellite networks have been proposed for providing communications for mobile users over a large geographical area where cellular service may be impractical. In these proposed systems, which include the European-African Telephone ("EAST") System and the Asian Cellular Satellite ("ACES") System, terrestrial-based cellular stations would be provided in high traffic areas, while an L-Band satellite communications network would provide service to remaining areas. In order to provide both cellular and satellite communications, the user terminal handsets used with this system would include both a satellite and a cellular transceiver. Such a combined system could provide full communications coverage over a wide geographic area without requiring an excessive number of terrestrial cells.
While a combined satellite/cellular network overcomes many of the disadvantages associated with traditional cellular systems, providing user terminal handsets for such systems that meet consumer expectations regarding size, weight, cost, ease of use and communications clarity is a significant challenge. Consumer expectations in this regard have been defined by the radiotelephones used with conventional cellular systems, which only require a single transceiver and which communicate with base stations that typically are located within 20 miles from the mobile user terminals. By way of contrast, the handheld user terminals which will be used in the integrated cellular/satellite systems include both a cellular and a satellite transceiver with the satellite communications subsystem having sufficient power and antenna gain to establish a link with a satellite that may be 25,000 or more miles away. Moreover, the satellite transceiver preferably should provide a quasi-hemispherical antenna radiation pattern (to avoid having to track the satellite) and should transmit and receive circularly polarized waveforms (to minimize the signal loss resulting from the arbitrary orientation of the transmit and receive antennas and to avoid the effects of Faraday rotation in the ionosphere). The satellite transceiver should also operate over the full extent of potentially widely separated transmit and receive satellite frequency bands.
In light of the above constraints, there is a need for handheld satellite transceivers, and more specifically, antenna systems for such transceivers, capable of transmitting and receiving circularly polarized waveforms which provide a relatively high gain quasi-hemispherical radiation pattern over separate, relatively broadband, transmit and receive frequency subbands. Moreover, given the handheld nature of the user terminals and consumer expectations regarding size and ease of portability, the satellite antenna system capable of meeting the aforementioned requirements should fit within an extremely small physical volume. These user imposed size constraints may also place limitations on the physical volume required by the antenna feed structure and any matching, switching or other networks required for proper antenna operation.
Microstrip or "patch" antennas are relatively small, low profile antennas that are well suited for various applications requiring a quasi-hemispherical radiation pattern. The radiating structure of a patch antenna consists of two parallel conducting layers separated by a thin dielectric substrate. Patch antennas are typically conformable to both planar and non-planar surfaces, inexpensive to manufacture and mechanically robust. The most common patch antennas are rectangular and circle patch antennas.
While patch antennas may be designed to provide the quasi-hemispherical radiation pattern necessary for mobile satellite communications, the bandwidth of these antennas is typically only a fraction of a percent of the carrier frequency and at most a few percent. As discussed above, certain emerging cellular and satellite phone applications have relatively large transmit and receive frequency bands, each of which typically exceed the bandwidth provided by conventional patch antennas. Furthermore, while there are methods that can be used to increase the bandwidth of these antennas, such as by increasing the height of the substrate, these methods may not be suitable for cellular/satellite systems due to constraints on the size of the antenna.
Moreover, the bandwidth over which conventional patch antennas effectively operate may also be limited by power transfer considerations. This limitation occurs because the power transfer between the antenna and the transceiver typically is not lossless due to reflections which arise as a result of imperfect impedance matching. If large enough this reflected power loss, which may be expressed in terms of voltage standing wave ratio ("VSWR"), may prevent the communications system from meeting its link budgets.
While it may be possible to match the input impedance of the patch antenna to the impedance of the interconnecting transmission line(s) from the transceiver, such a match will only occur over a small frequency range as the input impedance of a patch antenna varies significantly with frequency. Accordingly, even if a perfect match (i.e., VSWR=1.0) is not required, an acceptable match will typically still only be achievable over some finite bandwidth. This bandwidth may be less than the operating bandwidth required by emerging cellular and satellite phone applications. As such, impedance mismatches may also serve to limit the effective bandwidth of patch antenna systems.
Dual-band patch antennas are known in the art. However, these prior art antenna systems typically used two separate patch antennas, such as side-by-side patch antennas or stacked patch antennas, to solve the problem of communicating over widely separated transmit and receive frequency bands. Such solutions not only disadvantageously increase the size of the antenna system, but in the case of side-by-side patch antennas, also result in sub-optimal radiation patterns due to the asymmetry of the geometry of the patch in conjunction with the small ground plane. Accordingly, a need exists for a new, low profile, satellite phone antenna system that is capable of providing a quasi-hemispherical antenna pattern with positive gain over widely separated, relatively broadband, transmit and receive frequency bands.